1. Field of the Invention
This invention relates to metal detectors of the induction balance type and especially to such metal detectors which have the ability to discriminate between ferrous and non-ferrous metal objects and also which have the ability to compensate for ground effects caused by the presence of mineralized soil.
2. Discussion of Related Art
The use of metal detectors in searching for valuable objects has increased considerably in recent years, both as a hobby and as a serious means of finding relics from the past, etc. This demand has resulted in the development of the induction balance metal detector to a very sophisticated level. The induction balance detector is provided with a transmitter coil and a receiver coil. The transmitter coil radiates an alternating signal of a predetermined frequency which induces a current in the receiver coil. When a metallic element is disposed in the field of the transmitter coil, its presence causes a variation in the coupling between the transmitter and receiver coils and, accordingly, the received signal changes in both amplitude and phase. This change in the received signal is utilized to provided an indication of a metallic substance within the transmitter field.
One difficulty with induction balance metal detectors exists when such detectors are used in areas containing magnetite and other conductive minerals in the soil. These elements cause unwanted amplitude and phase changes in the received signal which can result in either false indications being made by the detector or the absence of indications when valuable objects are in the vicinity. It is possible to provide a coupling between the transmitted and received signals in order to null out these "ground effects". Examples of such nulling circuits include that shown in U.S. Pat. No. 4,099,116, issued July 4, 1978, to Tyndall. The Tyndall detector includes an impedance network connected between the transmitter and the received coils which phase relates the transmitted and received signals in order to vary the amplitude and phase of the received signal so that the ground effect signals can be nullified. Such devices are effective but are plagued with the inevitable need to be constantly retuned as the mineral content of the soil being traversed changes. U.S. Pat. No. 4,024,468, issued May 17, 1977, to Hirschi, includes a tuning control and trimmer capacitor which are set to provide a tuning signal of the same phase angle as a residual received signal produced by magnetic mineral soil in order to nullify that signal. Again, in order to be effective, the Herschi control would need to be constantly adjusted in order to continuously compensate for changes in soil composition.
A further difficulty encountered with early induction balance metal detectors was the lack of ability to distinguish between precious metals and "junk" metals. These latter consisting of pull tabs, foil, tin cans, etc. It is, however, known that such "junk" materials can be distinguished from precious metals such as gold, silver and copper by the differences in phase and amplitude of the received signal. Circuits have been suggested which attempt to capitalize upon these variations. For instance, U.S. Pat. No. 3,826,973, issued July 30, 1974, to Pflaum, shows a metal detector wherein in response to a first phase relationship between the excited coil voltage and the pickup coil voltage, and an aural tone of a first, predetermined frequency is derived to indicate detection of a ferrous body. In response to a conductive body being in the magnetic field between the excitation and pickup coils, the resulting phase displacement causes the frequency of the oral tone to shft from the frequency associated with the detection of a ferrous body. U.S. Pat. No. 3,872,380, issued March 18, 1975, to Gardiner, shows a metal detector wherein the detected signal is applied to a wave shaping circuit to derive short duration pulses which trigger a flip-flop. The transmitted signal is applied to a phase shifter and a wave shaping circuit for application to the same flip-flop. The output of the flip-flop is applied to an indicator circuit containing a meter. The meter is maintained mid scale during quiescent conditions and is deflected one way or the other depending on the phase relationship of the signals actuating the flip-flop. The above-mentioned U.S. Pat. No. 4,099,116, to Tyndall, also contains a discriminator capability. Feedback from the transmitter coil to the receiver coil is used to adjust the phase relationships thereof and thereby null out received signals indicative of junk materials. Also, the above-mentioned U.S. Pat. No. 4,014,468, to Hirschi, contains a discriminating control. This control changes the phase of the residual output signal of the received coil to set the quiescent operating point at a selected angle between about 0 and -90.degree. relative to the component of the received coil signal produced by the presence of magnetic mineral soil, and a tuning control adjusts the amplitude of the residual signal to an audio threshold level. As a result, amplitude differences in the received component can be used to discriminate between objects located by the meter detector.